1. Field of the Invention
This invention relates to an apparatus for arranging a plurality of articles in an array for testing. More particularly, the invention relates to an apparatus for arranging small discrete articles, such as sheet diffused diode chips, in an array to permit automated testing of the chips.
2. Discussion of the Prior Art
Small electronic devices such as diodes and transistors are usually electrically tested prior to final packaging by commercially available test apparatus. Such test apparatus usually includes an X-Y or an X-Y-Z indexing table and a probe head mounted above the table. The devices to be tested are located on the table. Testing the devices economically requires that they be located in an array on the table to permit the table to be automatically indexed in accordance with preselected steps to position the devices in sequence with respect to the probe head.
Most devices are manufactured in such an array on a wafer. These devices are tested and marked in a process referred to as wafer probing. After separation from the wafer the devices are then grouped into acceptable and defective ones.
Sheet-diffused diodes cannot be tested before they are separated from the wafer. Before being separated into individual devices, the sheet-diffused wafer constitutes a single large-surfaced diode with a single diode junction extending parallel to its two planar metallized surfaces. When this wafer is separated into individual devices or chips, each of these chips becomes a diode. However, parameters of each of these diodes, such as the reverse bias leakage current, may vary and in some cases they may fall outside of a desired range of values. Consequently, it is desirable to test each of these diode chips after they have been separated from the wafer. Testing the individual chips, however, has not been feasible up to now, partly because handling small devices is by itself, difficult and partly because of the special atmosphere wherein the chips are maintained after separation.
Conventional methods require subjecting the chips after separation to an etching and a drying operation. After the etching operation, drying takes place in a dry, inert atmosphere. Testing the chips is advantageously done immediately after drying, otherwise the normal moisture content in ambient air affects the test results. However, handling small chips in a dry atmosphere presents special problems. For instance, electrostatic forces on the chips increase the difficulty and the cost of handling and arranging the chips into an array.
In the past, instead of testing all chips collectively, a statistical approach has been used to determine whether all chips of a process lot can be assembled into the final product or whether they shall be rejected entirely. Because of the additional cost of final assembly a decision point for accepting or rejecting a given process lot is reached when approximately 20% of chips tested on a random basis are found to be defective. The actual decision point, of course, depends on the cost of the final packaging. Rejecting some process lots entirely results in substantial material losses, and a correspondingly increased manufacturing cost. If, however, all acceptable chips can effectively and economically be separated from defective ones, substantial material savings can be achieved.